The present invention relates to a light beam scanning device, and more particularly to a light beam scanning device for applying a light beam reflected and deflected by a reflecting-mirror light deflector to an object to be scanned, through an optical element having an entrance surface inclined at a prescribed angle to the direction in which the light beam falls on the optical element, for reading or recording an image or the like with high accuracy.
There is known an image scanning reading/reproducing system for applying a light beam to scan a recording medium with image information recorded thereon to photoelectrically read the recorded image information for thereby producing an image signal, and modulating a light beam with the image signal and scanning another recording medium such as a photographic photosensitive medium or the like with the modulated light beam to form a visible image thereon.
In the known image scanning reading/reproducing system, each recording medium is scanned by the light beam deflected by a reflecting mirror light deflector which has reflecting facets that are rotatable or vibratable at high speed.
One known light beam scanning device employing such a reflecting-mirror light deflector is illustrated in FIG. 1 of the accompanying drawings. The illustrated light beam scanning device scans a stimulable phosphor sheet S carrying recorded image information with a laser beam L to photoelectrically read the recorded image information. When a certain phosphor is exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, for example, the phosphor stores a part of the energy of the radiation. When the phosphor exposed to the radiation is subsequently exposed to stimulating rays such as visible light, the phosphor emits light in proportion to the stored energy. The phosphor exhibiting such a property is referred to as a "stimulable phosphor", and the stimulable phosphor sheet S is a sheet having a layer of such stimuable phosphor.
In the light beam scanning device, a laser beam L emitted from a laser oscillating tube 2 is deflected in the direction of the arrow A by a a reflecting-mirror light deflector, i.e., a polygon mirror 4 to pass through a scanning lens 6. The laser beam L is then divided by a half-silvered mirror 8 into a scanning laser beam L.sub.1 and a scanning laser beam L.sub.2, which go in different directions that are about 90.degree. spaced from each other.
The scanning laser beam L.sub.1 reflected by the mirror 8 scans the stimulable phosphor sheet S in a main scanning direction (indicated by the arrow B) upon rotation of the polygon mirror 4. The stimulable phosphor sheet S is fed in an auxiliary scanning direction (indicated by the arrow C) by a sheet feed mechanism (not shown). Therefore, the scanning laser beam L.sub.1 scans the entire surface of the stimulable phosphor sheet S two-dimensionally. In response to application of the scanning laser beam L.sub.1, the stimulable phosphor sheet S emits light in an intensity proportional to the image information recorded thereon. The emitted light is then applied to a photomultiplier 12 through a light guide 10 with its light entrance end disposed over the stimulable phosphor sheet S along the main scanning line thereon. The light received by the photomultiplier 12 is converted thereby into an electric signal, which will be displayed on a display unit such as a CRT or recorded on a recording medium such as a magnetic tape.
The synchronizing laser beam L.sub.2 that has passed through the mirror 8 falls on a reference grating plate 16 of a synchronizing signal generator 14, and is applied through a cylindrical light collecting rod 18 disposed behind the reference grating plate 16 to light sensors 20a, 20b mounted on the opposite ends, respectively, of the light collecting rod 18. The reference grating plate 16 has typically an array or linear pattern 22 of alternate slits and bars along the direction in which the synchronizing laser beam L.sub.2 sweeps. Therefore, the synchronizing laser beam L.sub.2 which positionally corresponds to the scanning laser beam L.sub.1 is applied as a pulsed light signal to the light sensors 20a, 20b. As a result, the light sensors 20a, 20b produce a synchronizing signal from the applied pulsed light signal, which is employed to accurately read the image information from the stimulable phosphor sheet S.
The polygon mirror 4 used as the light deflector in the light beam scanning device has reflecting facts each having its reflectivity variable dependent on the angle at which the laser beam L is applied to the reflecting fact. Therefore, the energy intensity of the laser beam L reflected by the polygon mirror 4 varies with time, and the scanning laser beam L.sub.1 which scans the stimulable phosphor sheet S in the main scanning direction of the arrow B is subject to so-called shading. As a result, it is impossible to obtain accurate image information from the stimulable phosphor sheet S. Likewise, the synchronizing laser beam L.sub.2 that has passed through the mirror 8 to the reference grating plate 16 also suffers shading, thus failing to produce an accurate synchronizing signal.